On-the-Road Holley Tuning

A once-in-a-lifetime opportunity befell three ’12 HOT ROD Power Tour® participants—the chance to have Holley Performance Products’ Senior Product Manager and carb tuner extraordinaire Jay McFarland personally tweak their engines as they drove from Michigan to Illinois. Each car had a carburetor that was either fresh out of the box or butchered by some clueless individual wielding a power drill and a screwdriver. Over the course of a few days Jay improved the driveability of each car using driver feedback, data from the new wideband oxygen sensor and in-car gauge, and power numbers generated by a mobile chassis dyno.

The tuning session began in the parking lot of GM’s Milford Proving Ground, where each car was dyno tested to see how much power the engines made and how rich or lean the engine ran at wide-open throttle. Then the Holley crew installed the new standalone wideband air/fuel ratio kit (PN 534-201, $379.95 at SummitRacing.com). The drivers spent the next few days keeping a written log of their engine’s fuel economy and air/fuel ratio as reported by the O2 system at different rpm points. Armed with this info, Jay tuned the carbs and engines until the cars were either dialed-in, we ran out of time, or out of tuning parts. We tagged along during this road-trip tuning session and convinced the guys from Dyno Tune to put each car on their mobile chassis dyno once again to see how much power each one put to the rollers after Jay’s tuning changes. The result was more power every time, and in the case of two cars, better mileage, too. One of the cars gave up some fuel economy in trade for more power but the owner was happy with that trade because he finally had a muscle car that performed like it looked. In the end, everyone drove home a bit more efficiently.

Day 1

Sonny reports that the Chevelle’s exhaust smells rich at idle and that the engine bogs off the line under wide-open throttle, which is odd because the O2 sensor ultimately proved the engine to be idling on the lean side. The carb has No. 71s in the primary and 80s in the secondary metering block, an 0.028-inch primary squirter and an 0.031-inch secondary squirter, and a 6.5 in/hg power valve.

Day 2

The first day of driving revealed that at wide-open throttle, the engine’s air/fuel ratio was a lean 14.0:1 at the bottom of the powerband and rich 11.8:1 at the top of the powerband. At part throttle from 25–55 mph, the engine ran from 13.1:1 to 14.0:1 A/F. It stayed near 14.0:1 A/F all the way to 80 mph. The engine idled at 15.0:1 with 9 in/hg of vacuum and base ignition timing was set at 10 degrees advanced. Fuel economy was 15.6 mpg during a combination of highway and in-town driving.

Changes: After observing the manifold vacuum reading with the engine idling and trans in gear, Jay made his baseline adjustments with the carb sitting on a workbench. Cutting the vacuum reading in half determined what size power valve should be in the primary metering block. In this case, he selected a 4.5 in/hg power valve (9/2=4.5). Since the wideband O2 gauge gave him the ability to see small tuning changes accurately, Jay upsized the primary jets just one size to 72s to richen the engine overall. Once the carb was back on the engine, Jay turned the idle mixture screws in the primary and secondary metering blocks counter-clockwise to richen the idle circuit, producing a 13.2:1 air/fuel ratio with the engine running and transmission in gear. He then advanced the base ignition timing to 12 degrees (which, of course, also advanced the total timing by 2 degrees). No changes were made to the accelerator pump squirters because the lean idle mixture could have caused the starting line bog. Jay wanted Sonny to drive the car again before making changes to see if this fixed the bog before adjusting the accelerator pump squirters. These changes increased idle manifold vacuum to 11 in/hg.

Day 3

No data was collected today because of transmission problems. Sonny had to stop and have a new 700R4 transmission installed so he could continue the road trip. He found a bargain at a local speed shop in Illinois and was back on the road.

Day 4

The previous carb tuning session resulted in an air/fuel ratio of 12.5:1 at 25 mph at part throttle, which gradually got leaner until the O2 gauge reported 13.9:1 a/f at 80 mph. Average fuel economy suffered slightly as Sonny drove in town and on the highway with larger primary jets. The Chevelle was getting only 14.4 mpg, but it was also running better overall. The second dyno test showed the engine was running at 12.2:1 A/F at the bottom of the powerband, 14.7:1 to 16.6:1 from 4,000 to 4,400 rpm (too lean, but there was not a chance to address that during Power Tour®), and a safe 12.5:1 to 11.17:1 from 4,600 rpm to 5,900 rpm. The 383 made 19 more peak horsepower and 40 lb-ft of torque and was better everywhere in the curve. The starting line hesitation did go away as well.

Day 1

Steve reported an overly rich running condition and starting line bog at wide-open throttle. His carb has been modified with drilled idle-restrictor passages in the primary metering block. It also had No. 72 primary and 80 secondary jets, an 0.028-inch primary squirter and an 0.031-inch secondary squirter, and a 5.5 in/hg power valve.

Day 2

The first driving test with the wideband O2 gauge installed showed the engine was running very rich. At part throttle, the engine had an air/fuel ratio of 11.3:1 at 25 mph and that continued until 65 mph, where it increased slightly to 11.7:1. The engine idled at 10.5:1. Initial ignition timing was set at 4 degrees advanced and idle vacuum was 7.5 in/hg. Average fuel economy was 10.5 mpg in a mixture of city and highway driving.

Changes: Jay, once again, observed the manifold vacuum reading at idle before removing the carb to tune it. He replaced the butchered metering blocks with new ones and switched the 5.5 in/hg power valve for a 5.0 in/hg power valve because he felt that at part throttle the valve wasn’t being held shut by the manifold vacuum and that was contributing to the rich running condition. The primary and secondary jets were reduced to 69s and 78s, respectively. Once the carb was back on the engine, Jay tweaked the idle mixture screws until the wideband O2 gauge showed the engine idling at 13.2:1 air/fuel with the trans in gear. Base timing was increased to 12 degrees advanced and the primary squirter was upped to 0.031-inch to help alleviate the starting line stumble. Due to time constraints, the ignition timing and squirter changes were made at the same time, so we don’t know which change affected the hesitation. The cumulative effects of the tuning increased the idle vacuum to 12 in/hg.

Day 3

The car still stumbled, but the air/fuel ratio leaned out nicely to 13.6:1 at 25 mph at part throttle. It gradually increased to 14.1:1 at 50 mph and then 14.2:1 at 70 mph and Steve allegedly continued accelerating to 110 mph, where the gauge read 14.1:1. The tuning changes resulted in a massive improvement in fuel economy: 17.43 mpg.

Changes: Jay felt the engine was a little lean up top, so the primary jets were bumped back up to the original 70s and based upon the now-higher manifold vacuum reading at idle, the 5.0 power valve was swapped for a 6.5. The primary squirter was increased once again to 0.035-inch with a hollow screw to aid flow, and both accelerator-pump cam arms were switched to the No. 2 position. Rotating the cam to the number 2 position increases the ramp, and the accelerator pump shot occurs faster with a slightly shorter duration.

Day 4

The Chevelle’s A/F ratio fell from 13.6 to 12.4 at part throttle at 35 mph and everywhere else in the cruise range. Average fuel economy also dropped to 12 mpg, likely the result of the power valve opening too early.

28/47Switch to 5.0 in/hg power valve

Changes: The power valve was changed back to a 5.0 and dyno testing showed Steve’s car picked up 10 hp and 16 lb-ft as low as 3,200 rpm and maintained that increase throughout the powerband. At the peak we saw 13.8 more horsepower at 5,900 rpm and 13 more lb-ft of torque at 5,000 rpm. The bog did go away. Steve called us after Power Tour® and said that cruising at 85 mph on the way home the car averaged 13.4-13.8 mpg

Drivetrain: TH400 with a stock converter, 12-bolt reared with 2.73 gears and a clutch-type Posi

Weight: 4,000 pounds (4,500 with the trunk full of luggage and tools)

Performance before tuning: 14.1 mpg

289 hp @ 4,900 rpm

343.12 lb-ft torque @ 3,800 rpm

Performance after tuning: 16.3 mpg

335.9 hp @ 5,100 rpm

375.7 lb-ft @ 4,000 rpm

Day 1

Tim reported several issues with his 454, included having to turn the primary throttle-blade idle speed screw several times to get the engine to idle. Once it did idle, it surged and dropped a lot of rpm when the trans was put in gear, which required another turn of the speed screw to make the engine idle at an acceptable rpm. The 454 bogged out of the hole and ran rich at wide-open throttle. The carb is stock and has No. 72 primary jets, a non-adjustable secondary metering plate, an 0.025-inch squirter, and a 6.5 in/hg power valve.

Day 2

At part throttle, from 25 mph to 40 mph, the engine ran at 11.8:1 to 12.8:1 air/fuel. The A/F climbed to 13.6:1 at 55 mph and dropped to 13.1:1 as the car accelerated to 70 mph. Tim observed the engine going lean for a second when he mashed the gas and passed other cars. At wide-open throttle, the air/fuel was at 13.6:1 at 3,000 rpm and richened as the rpm increased. By 3,500 rpm it was at 12.6:1, and hit 11.3:1 at 4,200 rpm. It flat-lined the oxygen sensor at 4,400 rpm with a dead-rich 10.0:1 air/fuel ratio. The engine idled at 12.8:1 with 15 in/hg of manifold vacuum. Fuel economy checked in at 14.1 mpg in a mixture of highway and in-town cruising.

Changes: With the carb on a workbench, Jay attacked idling problems by resetting the primary throttle blade position to stock specs and then opening the secondary throttle blades. Opening the secondary throttle blades 0.020-inch increased the idle speed without having the primary blades open too far. This was done by adjusting the setscrew under the choke side of the carb with a flat blade screwdriver. If you have one of those nifty 90-degree screwdrivers, it can be adjusted with the carb bolted to the intake, too. To address the rich WOT condition Jay decreased the primary jet size from 72s to 69s. To fix the lean spike when Tim mashed the pedal, Jay changed the secondary spring from a stiff black spring to a lighter silver unit to allow the secondary throttle blades to open sooner. The accelerator pump squirter was also increased to a 0.031-inch to alleviate the starting line bog. Jay then reinstalled the carb and adjusted the idle mixture screws to lean out the idle air/fuel ratio to 13.2:1. The engine idled a bit slowly even with the proper throttle blade adjustments, so Jay checked the ignition timing. Tim thought the timing was set at 14 degrees advanced, but that was because he checked it with the distributor vacuum-advance line connected to the carb, which resulted in a false reading. The problem was compounded because the primary throttle blades had previously been open so far to maintain an idle that there was vacuum above the throttle blades, which is right where the port for the distributor vacuum advance line was attached. Put simply, at idle, the vacuum advance system was working and increasing the ignition timing. Jay reset the timing with the advance line disconnected by turning the distributor and the engine finally ran better.

Day 3

The Impala ran much better and cruised at part throttle with 13.0:1 to 14.5:1 A/F numbers. The 454 big-block still stumbled when trying to accelerate from a stoplight and Tim said it was running a little rich under wide-open throttle based upon the Holley wideband O2 sensor readings. At a steady cruise, the O2 gauge showed a very lean 18.0:1 a/f when punching the gas pedal before settling down to very rich low 10s at wide-open throttle. Jay increased the accelerator pump squirter to 0.035-inch with a hollow screw for more fuel flow and moved the pump arm to the number two position to fix the bogging condition and the leanout at throttle tip-in. The wide-open richness was a problem that couldn’t be addressed at the time because this carb doesn’t have an adjustable secondary metering block with jets in it.

Day 4

The Impala gained 2.2 mpg in average fuel economy when driving around town and on the highway (averaging 16.3 mpg). The A/F ratio varied from 12.4:1 to 14.3:1 during part-throttle acceleration from 25 to 75 mph. The bog was lessened but still noticeable, so Jay switched the accelerator pump cam from the aggressive green colored part to the moderate orange cam for a longer duration accelerator pump shot and went with a larger 0.037-inch squirter. Those changes solved the hesitation. On the chassis dyno, the horsepower increased by 46 numbers, even though the wide-open throttle air/fuel ratio was a disappointing 10.0:1 from 4,400 rpm on up to 6,000 rpm. Once Tim returned home from Power Tour®, Jay sent him an adjustable secondary metering block conversion kit for his carb (PN 34-6), which bolts right on and has jets that can be tailored to his engine. Tim has since told us that the engine runs even better now with No. 72 jets in the new secondary metering block and its fuel consumption is better.